1. Technical Field
The present invention relates to a droplet ejection apparatus and an ejection failure recovery method.
2. Background Art
An ink jet printer, which is one type of a droplet ejection apparatus, forms an image on a predetermined sheet of paper by ejecting ink drops (droplets) through a plurality of nozzles. A printing head (ink jet head) of the ink jet printer is provided with a number of nozzles. However, at times, some of the nozzles are blocked due to an increase of ink viscosity, intrusion of air bubbles, adhesion of dust or paper dust, etc., and become unable to eject ink drops. When the nozzles are blocked, a missing dot occurs within a printed image, which results in deterioration of image quality.
Conventionally, as a method of detecting such an ejection failure of ink drops (hereinafter, also referred to as the missing dot), JP-A-8-309963 has disclosed a method of optically detecting when no ink drops are ejected through the nozzles of the ink jet head (ink drop ejection failing state) for each nozzle of the ink jet head. This method makes it possible to identify a nozzle causing the missing dot (ejection failure).
According to the optical missing dot (droplet ejection failure) detecting method described above, however, a detector including a light source and an optical sensor is attached to a droplet ejection apparatus (for example, an ink jet printer). Hence, this detecting method generally has a problem that the light source and the optical sensor have to be set (provided) with exact accuracy (high degree of accuracy), so that droplets ejected through the nozzles of the droplet ejection head (ink jet head) pass through a space between the light source and the optical sensor and intercept light between the light source and the optical sensor. In addition, such a detector is generally expensive, which poses another problem in that the manufacturing costs of the ink jet printer are increased. Further, the output portion of the light source or the detection portion of the optical sensor may be smeared by ink mist through the nozzles or paper dust from printing sheets or the like, and the reliability of the detector may become a matter of concern.
Also, according to the optical missing dot detecting method described above, the missing dot, that is, an ejection failure (non-ejection) of ink drops of the nozzles can be detected; however, the cause of the missing dot (ejection failure) cannot be identified (judged) on the basis of the detection result. Hence, there is still another problem in that it is impossible to select and perform adequate recovery processing depending on the cause of the missing dot. For this reason, sequential recovery processing is performed independently of the cause of the missing dot in the conventional missing dot detecting method. For example, ink may be pump-sucked (vacuumed) from the ink jet head under circumstances where a wiping process is sufficient for recovery. This increases discharged ink (wasted ink), or causes recovery processing of several types to be performed because adequate recovery processing is not performed, and thereby reduces or deteriorates throughput of the ink jet printer (droplet ejection apparatus).
Incidentally, the droplet ejection apparatus (ink jet head) generally includes a plurality of nozzles and actuators corresponding to the respective nozzles, and it is difficult for such a droplet ejection apparatus having a plurality of nozzles to detect an ejection failure (non-ejection) of droplets (ink drops), that is, the missing dot during a printing (recording) operation, without reducing or deteriorating the throughput of the apparatus.